This disclosure relates in general to the determination of a migration pathway of a subterranean fluid through a geological volume.
The characterization of subsurface strata is important for identifying, accessing and managing reservoirs. The depths and orientations of such strata can be determined, for example, by seismic surveying. This is generally performed by imparting energy to the earth at one or more source locations, for example, by way of controlled explosion, mechanical input etc. Return energy is then measured at surface receiver locations at varying distances and azimuths from the source location. The travel time of energy from source to receiver, via reflections and refractions from interfaces of subsurface strata, indicates the depth and orientation of the strata.
U.S. Pat. No. 7,248,539 discloses a method for automated extraction of surface primitives from seismic data. For example, one embodiment of the method of U.S. Pat. No. 7,248,539 involves defining, typically with sub-sample precision, positions of seismic horizons through an extrema representation of a 3D seismic input volume; deriving coefficients that represent the shape of the seismic waveform in the vicinity of the extrema positions; sorting the extrema positions into groups that have similar waveform shapes by applying classification techniques with the coefficients as input attributes using unsupervised or supervised classification based on an underlying statistical class model; and extracting surface primitives as surface segments that are both spatially continuous along the extrema of the seismic volume and continuous in class index in the classification volume.
The characterisation of faults and fractures in reservoir formations can also be important. For example, fractures intersecting drilled wells may assist the flow of hydrocarbons from the reservoir into the wells and so increase production. Conversely, fractures may allow water to flow into the wells and so decrease production.
WO 2008/086352 describes a methodology for mapping fracture networks from seismic data using fracture enhancement attributes and fracture extraction methods. For example, borehole data can be used to determine modes of fracture, and in particular whether fracture clusters or networks would be detectable in surface seismic data. It can also provide information on fracture network inclination (i.e., average inclination of the fractures in a network relative to the horizontal) and strike azimuth (i.e., average direction of intersection of the fractures in a network relative to the horizontal).
Discontinuity extraction software (DES), for example as described in U.S. Pat. No. 7,203,342, may then be utilised to extract 3D volumes of fracture networks from surface seismic data. Extracted fracture networks may be parameterised in terms of the strength of their seismic response, and on their length, height and width.
The approach of U.S. Pat. No. 7,203,342 may also be used to characterise and extract other geological features, such as faults, from seismic data.
Non-rigid matching, for example as disclosed in U.S. Pat. No. 6,574,563, is a methodology for analyzing the changes between two seismic surveys (time-lapse seismic). The generation of a 3D displacement vector field with subsample/subvoxel precision shows the displacement of the reflectors from one survey to another. A local varying matching function using a smooth displacement field allows for the comparison of two different seismic surveys.
Reservoir production and/or injection may compromise the cap rock integrity and cause leakage of fluid from a reservoir to the overburden. Escaped reservoir fluid may have severe consequences such as pollution of ground water or the ocean and may also be the cause of an unexpected drop in the pressure of the reservoir and hence a drop in production.
To prevent leakage problems, a risk assessment can be performed, with the geomechanical properties of the reservoir seal and overburden being taken into account. In some cases, the risk assessment may have to consider that the reservoir seal has been breached. Being able to evaluate the reason for such a breach, and determine the pathway the reservoir fluid has undertaken may enable mitigation of the consequences of the breach and development of solutions to the problem.
The use of neural networks to combine different seismic attributes to extract (among other things) possible gas chimneys acting as vertical migration paths for hydrocarbons is discussed in Meldahl, P., Heggland, R., Bril, B., and de Groot, P., The chimney cube, an example of semi-automated detection of seismic object by directive attributes and neural networks: part I; Methodology, SEG EXPANDED ABSTRACTS 18, 931-934 (1999); and Heggland, R., Meldahl, P., Bril, B., and de Groot, P., The chimney cube, an example of semi-automated detection of seismic object by directive attributes and neural networks: part II; Interpretation, SEG EXPANDED ABSTRACTS 18, 935-940 (1999). Combining faults and gas chimneys to detect hydrocarbon migration pathways and predict vertical seal and charge risk is discussed in Connolly, D. L., Brouwer, F. and Walraven, D., Detecting fault-related hydrocarbon migration pathways in seismic data: Implications for fault-seal, pressure and charge prediction, GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES TRANSACTIONS, Vol. 58: pp. 191-203 (2008). Using amplitude variation with offset (AVO) cubes and attributes derived from them to detect fault migration paths is discussed in Nyamapfumba, M. and McMechan, A., Gas Hydrate and Free Gas Petroleum System in 3D Seismic Data, Offshore Angola, GEOPHYSICS, Vol. 77(6): pp. O55-O63, 2012.